Oil and gas wells typically utilize a choke for controlling the flow from the production tubing. The choke has a choke element that moves within a choke housing to vary the flow area. One common type of choke, particularly for subsea production, has a stepping drive actuator for varying the position of the choke element. An actuator rod extends from the actuator into engagement with the choke element. The actuator typically has a gear train connected with a stepper motor, which when driven, causes rotation of a threaded drive nut. The drive nut engages a threaded portion on the actuator rod to move the rod linearly in response to the rotation. Chokes are constructed of materials that enable them to withstand erosion due to high flow rates and granular material within the well fluid.
Subsea production trees have a number of valves that can be opened and closed for controlling the well fluid. The valves are typically gate valves that operate either in a fully closed or fully open position. Because of the erosive conditions, gate valves do not serve as chokes. Typically subsea chokes do not serve as emergency valves because chokes close too slowly in the event of an emergency. If a subsea choke could effectively serve as an emergency shut off valve, fewer valves would be required on a tree.
One prior art solution to using a choke also as an emergency shutoff valve involves providing the choke actuator with a ball screw driven by a hydraulic piston to rotate the ROV (remote operated vehicle) override of the choke. That design, however, is complex and fairly large. Another solution is to utilize a hydraulic actuator with a bank of accumulators, however in deeper waters, the amount of accumulation required would be extensive. In another approach, U.S. Pat. No. 6,315,264 discloses a choke modified for rapid closure.